1. Field of the Invention
This disclosure relates to an image forming apparatus configured to superimpose toner images formed on a plurality of photosensitive members one on top of another by using a conveying member.
2. Description of the Related Art
An image forming apparatus configured to superimpose a plurality of toner images formed respectively on a plurality of photosensitive members by using a conveying member (an intermediate transfer member or a recording material conveying member) one on top of another is widely used. When exposure of an image from one scanning line to another is performed on a plurality of the photosensitive members, positional shift occurs in a main scanning direction and a sub scanning direction between a plurality of the toner images conveyed in a superimposed manner on the conveying member. Therefore, the image forming apparatus provided with a plurality of the photosensitive members performs a detection mode in which alignment toner images are formed on a plurality of the photosensitive members and are transferred to conveying members, and a plurality of the alignment toner images are detected on the conveying member by using an optical sensor, when an image is not formed.
For example, in JP-A-2001-134036, linear alignment toner images inclined from the main scanning direction of the photosensitive members by a predetermined angle are formed on a plurality of the photosensitive members and transferred to the conveying member. Depending on the result of detection of the alignment toner images transferred from a plurality of the photosensitive members to the conveying member, positions of the toner image to be formed on the respective photosensitive members in the main scanning direction and the sub scanning direction are adjusted.
In JP-A-2007-3986, linear alignment toner images inclined by different angles with respect to the sub scanning direction are formed on a plurality of the photosensitive members, are transferred to the conveying member, and are superimposed one on top of another. Depending on the result of detection of the alignment toner images superimposed on the conveying member, positions of the toner image to be formed on the respective photosensitive members in the main scanning direction and the sub scanning direction are adjusted.
In JP-A-2012-42875, an electrostatic image graduation including electrostatic image indexes arranged in parallel in the main scanning direction at regular intervals in the sub scanning direction is formed on a photosensitive member on an upstream-most side and is transferred to a conveying member. On a plurality of photosensitive members on a downstream side, the electrostatic image indexes formed on the photosensitive members and the electrostatic image indexes formed on the conveying member are aligned to adjust superimposition of the toner images in real time.
As described in JP-A-2007-3986, when the linear toner images are superimposed on the conveying member, positional information (or timing information) cannot be acquired individually from the respective linear toner images. Therefore, as described in JP-A-2001-134036, toner image scales, the positional information of which are acquired individually, need to be formed with appropriate shift in the main scanning direction so as not to be superimposed with each other. Therefore, a plurality of tracks for forming the toner image scale need to be formed on the conveying member in parallel, and hence a reduction in size of the photosensitive members or the conveying member is hindered.